The chains do not get merged. It is just, that the longest block chain is the consensus history of all transaction ( and it needs to be consistent). So if the .

The chains do not get merged. It is just, that the longest block chain is the consensus history of all transaction ( and it needs to be consistent). So if the .7 branch overtakes the .8 branch, then legitimate clients will detect that a transaction was not carried out and retransmit it. ( And the scam you are describing is precisely why Mt Gox did stop accepting bitcoins. They are afraid that someone can take advantage of the situation.)